Reactor product discharge system

ABSTRACT

A process is disclosed for removing particulate product from a fluidized bed reactor to optimize, within the constraints of the product removal cycle, conservation of gas in which the product is entrained. Data are compiled on the time required for equalizing pressure between product discharge tanks, and these data are used to determine the optimum times for various valves to be open. Product movement steps and equalization steps can be performed simultaneously.

FIELD OF THE INVENTION

This invention relates to the production of polyolefins in fluidizedbeds, and particularly to an efficient method of removing polymerizedproduct from a fluidized bed reactor at a desired rate of productionwhile optimizing the conservation of fluidizing gas.

BACKGROUND OF THE INVENTION

In the production of poloyolefins in fluidized bed reactors, a longstanding problem has been the loss of unreacted monomers and othercomponents of the fluidizing gas as a consequence of removing the solidproduct from the reactor. The polymer product is in the form of smallparticles and is generally removed by passing them, usually by positivegas pressure transfer assisted by gravity, while still more or lessentrained in, or at least surrounded by, the gaseous atmospherepredominating in the reactor. Regardless of the system of valves whichcauses the flow of solid product to the desired destination, the gasleaving the reactor along with the product generally contains asignificant volume of unreacted monomer. This unreacted monomerrepresents an economic loss in at least two ways—it is no longer in thereactor where it can form the desired product, and it represents anenvironmental and safety issue which must be dealt with. The fluidizinggas may include inert gases used, for example, to control the dew pointof the recycle gas. This also represents an economic loss. It istherefore desirable to minimize the amount of gas accompanying theproduct as it leaves the reactor. There is also a secondary effect,arising from the care taken to minimize the removal of gas with thefinished product—the systems developed to assure only a minimal loss ofgas tend to retard the removal of solid product, which can cause theentire process to be dependent on the rate of product removal ratherthan the otherwise possible rate of production. It is highly undesirablefor the production rate of a large, expensive reactor to be limited bythe product removal system.

In Aronson's U.S. Pat. No. 4,621,952, it is pointed out that theoriginal Union Carbide fluidized bed process for making polyolefinsincluded a gas lock zone associated with the product discharge train.Unreacted monomer accompanying the product resin was vented and recycledback to the reactor by compression. See U.S. Pat. Nos. 4,003,712,4,032,391, 4,255,542, and 4,302,565. The Aronson '952 patent describesthe use of a settling zone to fill a vessel with as much solid materialas is practical, thus minimizing the amount of gas in the settling zoneor vessel before it is sealed off by appropriate valving from the restof the system. It also describes a transfer zone downstream from thesettling zone. While discharging to the transfer zone, the settling zonereaches an intermediate pressure which is then preserved and can beincreased after the settling zone is emptied by connection to anothersettling zone containing higher pressure gas, to reduce the amount ofgas that can enter the settling zone from the reactor in the next cycle.

In U.S. Pat. No. 4,535,134, the loss of gaseous monomer during solidproduct removal from a horizontal reactor is reduced by controlling thepowder level in a receiving container.

SUMMARY OF THE INVENTION

Our invention is a method of product removal from a fluidized bedpolyolefin reactor which optimizes the conservation of monomer and othergas components within the time available for the product dischargefunction. The invention calls for a plurality (preferably two) parallelsets or series of vessels for receiving product from the reactor. Theproduct is passed, in each parallel train, first to a first stagevessel, then to a second stage vessel having a reduced pressure, andfinally to a conveyor, bin, or other end destination of a pressure lowerthan that of the second stage vessel. The conveyor, bin, or other enddestination may have a pressure lower than atmospheric—for example,where gas is drawn from it by a vacuum—but may also be atmospheric orhigher than atmospheric. Optionally, a third stage vessel of furtherreduced pressure may be inserted between the second stage vessel and thefinal destination. After each discharge from one vessel to another,connection is made between the just-filled vessel and the equivalent onein the other train, so that gas may pass from the higher pressure vesselto the lower pressure vessel. Appropriate valves are manipulated by acontrol system which takes into account the pressures and volumes of gasin both vessels over time, the effect (optionally) of solids content inthe vessel on the volume of gas in the vessel, the flow resistancecharacteristics of the connections between the vessels, and the overallproduct discharge cycle time. On-line determination of the flow and fillcharacteristics of the current product can be utilized with or withouthistorical data. The control system allocates a time for equalization ateach stage to permit the most efficient sequence of steps—that is, topermit the removal of all product made within the discharge cycle timeperiod, with a minimum or optimum level of gas loss. The sequence ofsteps includes as an option the simultaneous performance of certainvalve operations. That is, more than one step may be performed at thesame time, as will be seen below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a typical fluidized bed olefin polymerizationreactor showing the recycling of gas and the product removal system.

FIG. 2 shows the tanks and valving for a diagrammatic description of a10-step product removal procedure.

FIGS. 3a and 3 b show typical tank pair pressure equalization curves,together with the effects of valve closing before equalization isreached.

FIG. 4 shows product discharge times available for seven differentproducts at various production rates in a given facility.

FIG. 5 illustrates an 8-step product removal procedure in block form.

FIG. 6 is a diagrammatic sequence of six product removal steps.

FIG. 7 shows a 4-step product removal sequence.

FIG. 8 illustrates diagrammatically a sequence of steps for a 6-tankproduct removal system.

DETAILED DESCRIPTION OF THE INVENTION

This invention addresses the problem of maintaining optimal operation ofthe product discharge or removal system in a fluidized bedpolymerization reactor such as is used in the “Unipol” and otherprocesses, to maintain desired production rates while minimizing lossesof cycle gas. Optimizing performance of the product discharge system isaccomplished by adjusting the duration of each step within a productdischarge time cycle. Time allocations for the functions of interest aredetermined by the timing of opening and closing valves. The valves arepreferably controlled by a digital controller, which is programmed tofollow a specific sequence. The time spent in each of the steps in thesequence can be adjusted to provide for a desired rate of reactordischarge with minimal loss of cycle gas.

The usual or typical operation of a large fluid bed polyolefin reactorcalls for the establishment of a product discharge cycle time based onthe production rate for a similar or current product. Generally there isa physical or volumetric limit to the amount of product that can behandled in the product discharge equipment. But this is not to say thatthere is always a predetermined product discharge cycle time. Theproduct discharge operation and/or the sequence of steps for productdischarge may be determined without regard to a predetermined overallproduct discharge cycle time, and this variation of conducting theprocess is within the invention's scope or utility, although much of thefollowing description assumes an allotted amount of time for the productdischarge cycle, defined to be a series of steps for removing productwhich is used through a plurality of iterations. Where there is nopredetermined product discharge cycle time, our invention may still beused to conserve gas by controlling the passage of gas from vessel tovessel as described herein, and/or by simultaneously performing thevarious process steps described herein.

The invention includes a method of automating the process by whichadjustments are made in the discharge tank control procedure. Theinvention includes monitoring discharge tank performance, with automatedadjustment of the parameters needed for performance optimization. Theinvention therefore can ensure a high level of discharge tank systemperformance and at the same time free operating personnel for othertasks.

FIG. 1 shows the basic overall operation of the reactor, including basicfunctions of the prior art product removal system. In FIG. 1, reactor 1is a standard “Unipol” reactor, a type commonly used for makingpolyethylene, ethylene copolymers, and other olefin polymers. Thereactor 1 includes straight section 2, which typically extends 50 feetabove distribution plate 9, and expanded section 3. As is known in theart, the raw material monomer is continuously introduced in the gas orliquid phase, often near the bottom of the bed, shown schematically asline 4, and an appropriate catalyst from source 6 is introduced into thefluid bed at a point such as point 5. Cocatalyst and other additives maybe introduced from sources 22 and 23. Particles of polymer 25 are formedby the action of the catalyst on the monomer, suspended in reaction zone7, and withdrawn as product at one or two take-off points 8 usually neardistributor plate 9. Product withdrawal is usually initiated when theproduct inventory in the reactor exceeds a desired value. The inventoryin the reactor is generally determined by either the bed level or weightmeasurements, or from secondary measurements used to infer bed level orbed weight. Upon detection of a high reactor inventory, the productdischarge system is initiated to pass product to product discharge zones13 and/or 14, further explained below. Gas is continuously recycledthrough line 10 to a heat exchanger 11, where it is cooled and/orcondensed to remove the heat of reaction. In an alternate scheme notshown, gaseous feed monomer may be introduced to line 10 and make-upliquid in line 24; line 4 would not be used. Our invention may be usedwith other variations known to the art for conducting the polymerizationprocess in a fluid bed reactor.

Introduction of a nonreactant gas such as propane, butane, isobutane,pentane, isopentane, hexane, heptane, octane (saturated hydrocarbonshaving 3-8 carbons), or mixtures thereof into the system will help tocontrol the dew point of the gas phase in the reactor. See U.S. Pat. No.4,941,929 discussed above. Make-up gas, which generally consists of themonomer, nitrogen, and hydrogen, but may contain isopentane, propane, orother nonreactant gas, is fed to the reactor system at a rate to keep asteady state gaseous composition. The composition of the make-up gas isdetermined by a gas analyzer 12. The gas analyzer determines thecomposition of the gas being recycled and the composition of the make-upgas is adjusted accordingly, usually to maintain a desired gaseouscomposition within the reaction zone 7. The dew point may also becalculated using the gas analyzer (or a separate analyzer) readings oroutput in a known manner.

The polymer production rate of the bed is controlled primarily by therate of injection of the catalyst components and monomer. Theproductivity of the bed may also be modified by increasing the rate ofinjection of the components and decreased by reducing the rate ofinjection.

Removal of the particulate product through a take-off point 8 ispreferably controlled by a pair of timed valves which may alternatelyplace product in discharge zones 13 and 14, discussed in more detailelsewhere herein. Under a given set of operating conditions, thefluidized bed is maintained at a substantially constant height or weightby withdrawing a portion of the bed as product at a rate equal to therate of formation of the particulate polymer product. Since the rate ofheat generation is directly related to product formation, a measurementof the temperature rise of the gas across the reactor (the differencebetween inlet gas temperature and exit gas temperature) is determinativeof the rate of particulate polymer formation at a constant gas velocity.

As indicated elsewhere herein, the product removal zones 13 and 14 aredesigned to handle certain amounts of product within a given period oftime, while conserving as much gas as practical.

In FIG. 2, the ten-step sequence of the prior art is seen to be improvedand optimized by our method. For purposes of explanation andillustration, assume all valves A, B, C, D, E, F, G, and H to be closed.Illustratively, the sequence is begun by a control signal opening valveB or C. The solid product flows from the relatively high pressurereactor I into tank J or K, having a lower pressure, beginning movementof the solid particulate product through the right or left series ofvalves and tanks, as depicted, which may also be called the East-sideand the West-side, seen in FIG. 1 as product discharge zones 13 and 14.Sometimes tanks J and K may be referred to as product discharge tanksand tanks L and M may be called blow tanks.

Assuming that it was valve B that was opened to fill tank J withproduct, tank J now has a pressure approximating that of reactor I.Valve B is now closed and valve A is opened, allowing the pressures intanks J and K to approach equilibrium. Valve A is then closed and valveD is opened, allowing the solid material to flow from tank J, where ithad previously been deposited by the operation of valve B, into tank L.In addition to moving solid product, this tends to equilibrate the gaspressures in tanks J and L. Valve D is then closed and valve E isopened, allowing the pressure in tanks L and M to approach equilibriumby moving gas from tank L to tank M. Valve E is then closed and valve Gis opened, allowing the solid to flow from tank L to the product bin Nor other destination having a lower pressure, which may be a conveyorhaving a controlled pressure.

The complete discharge cycle will include, after discharge of product,the similar movement in turn of product through the westside or theeast-side, whichever was used only for gas pressure equalization duringthe first five steps. Thus, after the system moves product through tanksJ and L as described, additional solid product would now be moved fromreactor I to tank K by opening valve C, valve C would then be closed andvalve A would be opened to allow tank K to approach gas pressureequalization with tank J. Valve A would be closed and valve F opened tocause product to flow into tank M, valve F is closed and E opened tocause gas to flow into tank L from tank M; valve E is then closed and His opened to send product to destination P. Thus there are five stepsfor each of the west-side and the east-side three product-moving stepswith pressure equalizing steps between the first and second, and thesecond and third.

An optional feature shown on only one side of the product dischargesystem is a pipe 16 running between product discharge tank J and blowtank L. Usually most of the product moves quite readily because itsmovement is assisted by a considerable pressure difference. When tank Lapproaches full, however, and the tank pressures approach equal, productmovement depends more on gravity, and gas flow may tend actually toreverse as the displacement of gas by product becomes a significanteffect. At this point, valve 15 may be opened to permit displaced gas toflow from blow tank L to product discharge tank J. By permitting thedisplaced gas to flow through line 16, solid product falling from tank Jto tank L does not have to overcome a counterflow of gas moving upwardsfrom tank L, and the solid product movement will be accomplished soonerthan otherwise would be the case. It is desirable not only that theproduct be moved as quickly as the equipment permits, but that thereceiving tank be as full as possible. The operation of valve 15 may bein response to a signal representing a predetermined pressure differencebetween tanks J and L. A similar line and valve can be run between tanksK and M, between reactor I and tank K, and/or between tank J and reactorI. Such lines may be used on both sides of the discharge system. Also,movement of solid product through valve G and/or H may be assisted,particularly near the end of the operation, by adding gas to tank L or Mfrom a source having a higher pressure than the current pressure indestination N or P.

Should the cycle be interrupted for any reason, the valves may eprogrammed to empty the entire discharge system, as normally it is otdesirable for the product to remain quiescent together with gas (it ancontinue to react) in the tanks for extended periods of time. Aninterlock system may program the discharge of the product in an orderlymanner.

A measure of the efficiency of the operation of the system is the amountof gas which exits the system through valve G or H when the productleaves the system. The loss of gas is minimized by allowing the J and Ktanks as well as the L and M tanks to equalize in pressure at theappropriate times during the cycle. The degree to which the tanksequilibrate is in direct relation to the amount of time the equalizationvalves A and E are open. The time the equalization valves are open is,however, limited by the time available for the entire dischargesequence. Gas conservation is also promoted by assuring that as muchproduct as possible is placed in the tanks at each step, thus minimizingthe volume available in the tank for gas, as will be discussed furtherwith respect to FIGS. 3a and 3 b. The invention therefore minimizes gaslosses from the discharge system while providing for a completedischarge cycle within a prescribed time period.

Persons skilled in the art will recognize that equalizing pressures intanks J and K (and, at a different stage, L and M) establishes apressure in tank J, for example, lower than that of reactor I after it(tank J) is loaded with product, so that when solid product isdischarged from tank J into tank L, less gas will move to tank L thanwould be the case if tank J still retained the pressure of reactor I. Atthe same time, a higher pressure is created in tank K by theequalization, thus reducing the amount of gas able to pass from the highpressure reactor I to tank K when product is later moved into tank K.Without such equalization, product delivered to tank L, for example, inthe presence of relatively high pressure gas will take with it (throughvalve G) a large amount of gas. This would be particularly true whereend destination N or P is at a negative pressure, which is sometimes thecase.

While this type of sequencing in the past has performed to conserve gasand unreacted monomer component, the practice has not been optimized toconserve the most gas possible within the constraints of the increasingproduction of the reactors and processes used in them. Product removalmust keep up with production and there is a need for a method tooptimize gas conservation within variable product removal cycles,depending on the type of product, the production rate, and many otherfactors.

The invention includes manipulation of the parameters of the plantautomation systems. Referring again to FIG. 2, to illustrate thisprocess, consider all valves to be initially closed.

During one or more sequences of valve openings and closings describedabove, the following information is recorded—(i) the time valve A isopen, (ii) the pressures in each of the tanks J and K as a function oftime and, optionally, (iii) the solid levels in the tanks J and K as afunction of time. Pressure and time are recorded and compiled in a database for gas flow in both directions, and similar readings are taken forthe steps involving gas passing between blow tanks L and M. The data arethen analyzed to determine the operating characteristics of the system.The data are regressed to fit a first principles model that describesthe operating performance during the firing sequence. The modelparameters are used to determine the time parameters (including theduration of the timed steps) of the control system. This is thepreferred method. The first principle model describes the time varyingflow of cycle gas between the product discharge tanks J and K throughvalve A and between blow tanks L and M through valve E. the applicablemodel equations are

d P₁/dt=a*X and d P₂/dt=b*X, with X={square root over (P₁+L −P₂+L )}

Where t=time, a=a parameter representing the volume of gas in the highpressure tank and the resistance to flow between the two tanks, b=aparameter representing the volume of gas in the lower pressure tank andthe resistance to flow between the two tanks, P₁=the pressure in thehigh pressure tank, and P₂=the pressure in the lower pressure tank

Parameters a and b are determined by regression of the pressure versustime data. These parameters can be used to determine the efficiency ofgas flow in the system as well as the fill efficiency. It may beobserved that in a given facility the resistance to flow between twotanks will differ depending on the direction of flow, and this may betaken into account. Pattern recognition techniques or software may alsobe used to determine the product removal cycle performance relative toan ideal.

Data representing solid levels and/or weights in tanks are used with themodel parameters to calculate the amount of gas in the solid containingtanks, as well as the amount of product. These will be affected by theresin properties, including shape of the particles, their density,particle size, and porosity. A high bulk density will leave less openvolume for the gas than one not so closely packed. As will be seen inthe discussion of FIG. 3, the quantity of gas in the high-pressure,solid-filled tank will have a significant effect on the ultimateequalization pressure achievable with a paired “empty” (no solidproduct) tank.

Fill efficiency is a measure of whether the tank is as full as possible,given the resin bulk density. Fill efficiency may be inferred partlyfrom the pressure changes; the final pressure equalization point is anindication of the volume of product in the “full” tank, as the volume ofproduct is inversely related to the volume of gas at the higher pressurein the tank. Optionally, the current analysis information may becataloged with historical information describing discharge tankperformance.

Using the historical (optional) and current cycle information, completethe following steps—(a) estimate the total amount of time likelyavailable for the next product discharge cycle, based on currentconditions and the particular product being manufactured, and,optionally, historical data, such as compiled and presented in FIG. 4,which reflects the process model parameter and production rate history;(b) adjust control system timers to minimize cycle gas loss in theexpected time available for the next discharge tank cycle. This can bedone using the parameters of the fitted model or pattern recognition.Preferably, a product movement valve such as valve D, F, G, or H is heldopen only as long as is necessary to move all the available product, inorder to maximize the time available for the pressure in the two tanksabove the valves to equalize. The time for the lower tanks toequilibrate can be adjusted in the same manner. FIGS. 3a and 3 billustrate the influence of this time adjustment. Updated parameters maybe sent to the discharge tank control system for application to the nextcycle of the discharge system. Repeat the steps above.

Referring now to FIG. 3a, tank pressure is plotted against time fortanks similar to tanks J and K of FIG. 2, connected by a pipe similar tothe pipe which is intercepted by valve A. If tank J is at 307.65 psi atthe beginning of equalization—that is, when valves B, C, D and F areclosed and valve A is opened, and tank K is at 150.5 psi, an equalpressure of about 220 psi will be achieved in about 54 seconds for theconfiguration of tanks, valves and pipes tested, where curve 20represents pressure in tank J and curve 21 represents pressure in tankK. However, if the overall schedule for product removal will not permitas long as 54 seconds for the equalization step, curves such as curves20 and 21 will determine the progress towards equalization at any pointin time it is decided to terminate the equalization step. Curveextension 22, for example, shows the pressure in tank K if theequalization is stopped at about 20 seconds. Curve extension 23 showsthe pressure in tank J after valve A is closed and product is dischargedfrom tank J into tank L. As expected, the pressure in tank J is lowerthan that of tank K (extension curve 22), since tank J is now connectedto tank L, which has a lower pressure. While tank L was earlierdischarged to the product end destination, valve G is closed, so we mayexpect tanks J and L to be at about the same pressure Thus, a thirdsequential tank may be included (see FIG. 8) to further reduce gas loss.

For FIG. 3b, a different set of parameters was placed in the computermodel. Here the pressure in the high pressure tank represented by curve20 b begins at 298.2 psi and the lower curve 21 b begins at 145.25 psi.Pressure equalization at about 214 psi is achieved in about 54.4seconds, the pressures in the two cross-tied tanks following the curves20 b and 21 b following the pattern depicted. Curve extension 23 b showsthe pressure in tank J after valve A is closed and product is dischargedfrom tank J into tank L. It will be observed that in both FIGS. 3a and 3b the equalization pressure is not the average of the beginningpressures; accordingly the equalization pressure is a function of thequantity of gas in the two vessels, while the time required forequalization is determined by the resistance to flow between the tanks,the amount of resin in the tanks, the physical characteristics of thegas, and the initial pressure difference. Thus one may compile dataincluding such variables sufficient to impute the quantity of gasinitially in the high pressure tank from a comparison with a similarcurve followed by a current equalization step. One will also be able topredict the effect of stopping the equalization at a given point intime, as shown by curves 22, 23, and 23 b. Thus our invention includes aprocess for passing solid particulate product in the presence of gasfrom a fluidized bed through successive containers to control thepassage of gas along with the solid particulate product, wherein gassubstantially free of solid product is passed from a high pressurecontainer containing solid particulate product to a low pressurecontainer substantially empty of solid particular product, comprisingpassing the gas for a time period which is a function of gas flow ratesbetween the high pressure and low pressure containers. As mentionedpreviously, fill efficiency may be inferred simply from pressurereadings versus time in an equalization step, as the amount of gas inthe higher pressure tank is inversely related to the volume of solidparticulate product it contains. In the example of FIG. 3a, the fillefficiency was calculated to be 77%. For the Example of FIG. 3b, it wascalculated to be 78%. Thus our invention includes a method of estimatingthe amount of solid particulates in a first vessel containing solidparticulates together with gas at a first pressure, comprising (a)passing gas from the first vessel to a second vessel through aconnection of known pressure drop characteristics, the second vesselcontaining gas at a second pressure which is lower than the firstpressure (b) while the gas is passing from the first vessel to thesecond vessel, monitoring gas pressure in at least one of the vessels(c) estimating the volume occupied by gas originally in the first vesselat the first pressure by comparing the (at least one) monitored pressureover time to the known pressure drop characteristics, and (d) estimatingthe amount of solid particulates in the first vessel by subtracting thevolume of gas originally in the first vessel at the first pressure froma known volume of the vessel.

Referring again to FIG. 4, a family of curves is presented representingproduction rate versus the time required for product removal efficiencyfor seven different products to be made in a given facility. The curvesvary with the product resin characteristics, which include not only themonomer content, but many other factors such as molecular weight,density, and catalyst. The curves illustrate that some resins can bedischarged faster than others, but in each case the inefficiency of theproduct removal system can be measured in terms of time. For example,for the uppermost curve, a production rate of 60,000 pounds per hourrequires a discharge time for both sides of the system of 270 seconds.Accordingly, optimization must be accomplished within 270 seconds forthe product of the uppermost curve produced at a rate of 60,000 poundsper hour. It should be kept in mind that the primary purpose of theproduct discharge system is to remove product, and therefore it is notnormally desirable to cut short the operations of steps which moveproduct from one vessel to another; it is preferable, rather, to hastenthe completion of such steps, such as by the use of a line such as line16 as explained in connection with FIG. 2. More often, however, and inaddition to any other adjustments, our invention contemplates cuttingshort the passage of gas during an equalization step at a point in eachequalization step which is as long as desirable to optimize the passageof gas in all such steps. Normally this will mean allocating a timeperiod for each gas pressure equalization step which is less than thatwhich would achieve full equalization.

Looking now at FIG. 5, an 8-step sequence for the configuration of FIG.2 is seen to be presented diagrammatically. The block diagram of FIG. 5describes a series of steps 1-8. In step 1, product is discharged fromthe reactor I to tank J while another batch of product is moved fromtank M to an end destination having a relatively low pressure, such as abin or a conveyor. Each of these steps is isolated from the rest of thesystem by closing the appropriate valves. That is, referring to FIG. 2,only valves B and H are open during this step; all the others areclosed. In step 2, only valve A is open to allow gas to flow from tank Jto tank K, following a curve similar to that of FIG. 3a or 3 b. How longvalve A is open is determined by the above described control factorssuch as fill efficiency, cycle time, and gas flow resistance, tooptimize the overall optimization of gas in this and all other stepswithin the cycle time. For step 3, all valves are closed except valve D,so that product may move from tank J to tank L. In step 4, (again, seeFIG. 2) valve E is open to equalize, as far as the allowed time permits,the pressures in tanks L and M, while all other valves are closed. Step5, then, is similar to step 1 in that the reactor I now dischargesproduct into tank K while valve G is opened to deliver product from tankL to a destination outside the system. Thereafter (step 6), tanks J andK are equalized in pressure for the time period the controller permitsvalve A to be open, gas flowing this time from tank K to tank J; all theother valves are closed. Then, in step 7, product is moved from tank Kto tank M in a manner mirroring step 3, and lastly step 8 equalizespressures in tanks L and M in a manner similar to step 4 except that thegas moves in the opposite direction. It will be seen that there are fouropportunities in this series of steps to control the duration ofequalization—two at each tank stage. The optimum overall procedure mayinclude longer valve openings between tanks J and K, and between tanks Land M because there are different amounts of gas to be saved and thepressure differences vary; however, this will depend also on differencesin the particular equilibrium curves for the tank pairs—as mentionedabove, resistance to gas flow may vary with the direction of gas flow,and accordingly, for example, the time allocated for gas flow from tankJ to tank K in step 2 may be different for the opposite flow in step 6.

The six-step sequence of FIG. 6 permits additional simultaneous steps,thus providing more time for each step within the overall cycle timedetermined by the production rate. As seen in step 1, pressureequalization may be performed between tanks L and M (with gas flowingfrom tank M to tank L) while product is moved from reactor I to tank J.Tanks J and K are equalized in step 2 while product is simultaneouslydischarged out of the system from tank M. In step 3, product moves fromtank J to tank L. Step 4 is the mirror image of step 1 in that thereactor I discharges to tank K while tanks L and M are equalized, thegas moving from left to right, as depicted. Step 5 mirrors step 2, andstep 6 mirrors step 3, after which step 1 repeats. Throughout all steps,the only open valves are those which permit the desired product movementor pressure equalization.

The four-step sequence of FIG. 7 combines more steps and greatly reducesthe time required for product removal through the system. Here, in step1, product moves from the reactor I to tank J and from tank L to the enddestination on the “west”-side while, on the “east”-side, product movesfrom tank K to tank M, all valves not contributing to these movementsbeing closed. Then, in step 2, both sets of tanks—J and K, and L andM—are equalized in pressure by moving gas from the higher pressure tankto the lower pressure tank, each set of equalizing tanks being isolatedfrom the rest of the system by closed valves. Step 3 mirrors step 1 inthat product is moved from the reactor I to tank K, from tank J to tankL, and from tank M to the bin, conveyor or other destination outside thesystem. Step 4 is similar to step 2 in that both levels of tanks areequalized in pressure. In each step, as before, all valves are closedwhich do not accomplish the desired product move or equalization. Thefour-step process, requiring minimum time for product removal, permitscontrol of the steps' duration to be based more on product propertiesand equipment limitations, and less on an allotted cycle time based onproduction rate, and allows the longest time for gas pressureequalization.

A modification of the sequence just described will further enhance gasconservation. It frequently happens that, because the pressure is lowerin the lower tanks than it is in the upper tanks, product moves faster,in step 1, into tank J and out of tank K, than it does out of tank L. Itis therefore possible to begin the pressure equalization in tanks J andK, as depicted in step 2, before terminating the discharge of tank L,depicted in step 1. This permits a longer time for pressure equalizationin tanks J and K, after step 2 is fully entered into, than otherwisewould be the case, meaning that the termination of equalization can beperformed at a point further to the right on the curves of FIGS. 3a and3 b. A similar early initiation of equalization of tanks J and K canbegin before step 3 is completed, again permitting a longer time forequalization and conserving more gas than would otherwise be possible.

Further savings may be realized by adding a third stage of tanks asshown in FIG. 8. Here there will be three separate pressure zonesbetween the reactor and atmospheric or other lower pressure on eachside. One may move product in a manner similar to that described above,equalizing pressures in the opposite tank after each discharge, as shownin the block diagram of paired parallel tanks Q and S receiving productfrom reactor R and passing it, respectively, through tanks T, U, and V,W. In step 1, product is simultaneously moved from reactor R to tank Q,from tank S to tank V, from tank T to tank U, and from tank W to the enddestination. In step 2, all pairs of tanks on the same stages areequalized in pressure. Step 3 is the mirror image of step 1, and step 4mirrors step 2.

A fourth level of tanks may be added—steps 1 and 3 would be the productmovement steps, and steps 2 and 4 the gas pressure equalization steps,similar to the illustrations of FIG. 8, including all four levels orstages.

It will be appreciated that in all variations, the conservation of gasis accomplished by minimizing the pressure differences between thedischarging tank and the receiving tank, or, in the case of the enddischarge, between the discharging tank and atmospheric or otherpressure lower than that of the last tank. Our invention also enablestiming of the equalization steps within the constraints of an overallproduct delivery schedule which may be determined by the desiredproduction rate and/or the reactor bed level or bed weight and/or theresin properties. Thus our invention includes a method of controllingproduct discharge from a fluid bed polymerization reactor comprisingmonitoring bed level or bed weight or both (herein referred to as bedlevel/weight) of solid particulate product in the fluid bed, generatinga signal representing bed level/weight of solid particulate product inthe fluid bed, calculating a total discharge cycle time based on thatsignal, allocating portions or phases of the total discharge cycle timesequentially to each of a plurality of product discharge steps, andinitiating product discharge in each of the product discharge stepssequentially.

Thus it will also be appreciated that we optimize gas conservation inthe context of the product discharge process, the primary objective ofwhich is to remove the required amount of product within the designatedtime cycle, while we can minimize gas loss within the time available todo so. Our use of the terms optimum or optimize and minimum or minimizeshould be understood with the primary objective of product removal inmind. Our invention enhances the conservation of gas in the productdischarge process.

What is claimed is:
 1. A method of removing solid particulate productfrom a fluidized bed reactor, together with gas from the fluidized bedreactor, through two parallel series of tanks comprising a first productdischarge tank, a first blow tank, a second product discharge tank, anda second blow tank, to conserve the gas, the process comprising: (a)transferring the solid particulate product together with the gas fromthe reactor into the first product discharge tank; (b) while performing(a), transferring solid particulate product from the second productdischarge tank to the second blow tank; (c) while performing (a) and(b), transferring solid particulate product from the first blow tank toa further destination having a pressure lower than the first blow tank;(d) after performing (a), (b), and (c), passing gas from the firstproduct discharge tank to the second product discharge tank; and (e)passing gas from the second blow tank to the first blow tank, wherebyeach of (a), (b), (c), (d), and (e) are isolated by closed valves fromany of the reactors, the product discharge tanks, and the blow tanks notnecessary for the performance thereof, and whereby carrying out (a)-(e)constitutes a product discharge cycle.
 2. The method of claim 1 whereinthe duration of each of (a)-(e) is allocated within an overall productdischarge cycle time to optimize the conservation of gas in the reactor.3. The method of claim 1 wherein each of the parallel series of tanksincludes a further downstream blow tank and, wherein the furtherdestination in (c) is the further downstream blow tank.
 4. The method ofclaim 1, further comprising simultaneously transferring the solidparticulate product together with the gas from the reactor into thesecond product discharge tank, transferring solid particulate productfrom the first product discharge tank to the first blow tank, andtransferring product from the second blow tank to a further destinationhaving a pressure lower than the second blow tank, and thereafterpassing gas from the second product discharge tank to the first productdischarge tank and passing gas from the first blow tank to the secondblow tank.
 5. The method of claim 4, whereby the process recited thereinis repeated at least one time.
 6. The method of claim 4 wherein thesolid product discharge cycle is conducted in a predetermined timeperiod, and (d) and (e) are limited to time periods less than thosewhich would achieve substantial pressure equilibration in the respectiveproduct discharge tanks and blow tanks.
 7. The method of claim 1 whereinthe process of passing gas in (d) is initiated prior to the terminationof (c).
 8. The method of claim 7, further comprising simultaneouslytransferring the solid particulate product together with the gas fromthe reactor into the second product discharge tank, transferring solidparticulate product from the first product discharge tank to the firstblow tank, and transferring product from the second blow tank to afurther destination having a pressure lower than the second blow tank,and, prior to the termination of transferring product from the secondblow tank to a further destination, but after completion of the transferof product, initiating the passing of gas from the second productdischarge tank to the first product discharge tank to approach equalpressures in the product discharge tanks.
 9. The method of claim 8,whereby the process recited therein is repeated at least one time.
 10. Aprocess for removing solid particulate product and gas from an olefinpolymerization reactor, the reactor having a product discharge systemcomprising first and second product discharge tanks and first and secondblow tanks, the process comprising: (a) simultaneously transferringsolid particulate product and gas from the reactor to the first productdischarge tank and passing gas from the second blow tank to the firstblow tank; (b) simultaneously passing gas from the first productdischarge tank to the second product discharge tank and transferringproduct from the second blow tank to a further destination; and (c)transferring product from the first product discharge tank to the firstblow tank, each of (a)-(c) being isolated by closed valves.
 11. Themethod of claim 10, further comprising: (d) simultaneously transferringsolid particulate product and gas from the reactor to the second productdischarge tank and passing gas from the first blow tank to the secondblow tank; (e) simultaneously passing gas from the second productdischarge tank to the first product discharge tank, and transferringproduct from the first blow tank to a further destination; and (f)transferring product from the second product discharge tank to thesecond blow tank, each of (d)-(f) being isolated by closed valves. 12.The method of claim 11, whereby the process recited therein is repeatedat least once.
 13. The method of claim 11 wherein the amount of timerequired to pass the gas is that required to optimize the conservationof gas within a predetermined time period for the completion of all of(a)-(e).